1. The Field of the Invention
The present invention relates to the purification of ammonia. More particularly, the present invention provides methods, materials and apparatus for removing oxygen from ammonia. More specifically, the present invention provides a novel getter material that removes oxygen from ammonia at low temperatures.
2. Background
Pure gases are essential in a number of fabrication processes important in the semiconductor manufacturing industry. Gas purity is positively correlated with the yield of integrated circuits in semiconductor manufacturing and is especially critical in the fabrication of advanced modern transistors which are easily damaged by small amounts of contaminants because of their small feature widths.
Ammonia (NH3) is a process gas of primary importance in the semiconductor industry for the formation of nitride layers in electronic transistors through chemical vapor deposition and epitaxy processes. More specifically, ammonia is most commonly used for the formation of silicon nitride and silicon oxynitride films by direct nitridation of silicon oxide. Silicon nitride is both a more effective barrier to alkali ion migration and more resistant to thermal oxidation than silicon oxide and is thus particularly useful as a cover layer in metal-on-silicon (MOS) technology and as a mask when selective oxidation of the semiconductor is required. Silicon oxynitride films have physical properties intermediate between silicon and silicon nitride and have enormous potential for use in ultra large scale integrated circuits and as a passivating layer in gallium arsenide materials. Growing films of silicon nitride and silicon oxynitride requires ammonia of very high purity. Oxygen is a particularly harmful contaminant because its high chemical reactivity leads to its ready incorporation as an impurity into film during thermal nitridation of silicon oxide.
Getter materials comprise metals and metal alloys that sorb various gas molecules such as carbon oxides (COx, where x is 1 or 2), hydrogen (H2), nitrogen (N2), oxygen (O2) and water (H2O) and thus have been widely used for the purification of gases. In this application, advantage is taken of the ability of getter materials to sorb certain various compounds preferentially from gaseous mixtures. For example, such uses include the removal of carbon oxides (CO and CO2), nitrogen (N2), methane (CH4), and oxygen (O2) from hydrogen. The gettering function, and even the particular species that are sorbed, depends on the temperature of the getter materials.
Particularly useful are non-evaporable getter (xe2x80x9cNEGxe2x80x9d) materials, which include zirconium or titanium-based alloys in combination with elements such as aluminum, vanadium, iron, nickel or other transition elements or their combinations. Examples of getter materials include the alloy having the composition Zr 84%xe2x80x94Al 16% by weight, which is manufactured and sold by SAES Getters S.p.A. (Milan, Italy) under the name xe2x80x9cSt 101(copyright)xe2x80x9d, and the alloy having the composition Zr 70%xe2x80x94V 24.6%xe2x80x94Fe 5.4% by weight, also manufactured and sold by the SAES Getters under the tradename xe2x80x9cSt 707xe2x80x9d.
The use of getter materials to remove oxygen from ammonia has been disclosed, for example, in European Patent No. 484-301-B1 assigned to the SAES Getters and incorporated herein by reference for all purposes, which describes contacting oxygen containing ammonia with the getter alloy St 707 that selectively absorbed oxygen from the gaseous mixture. However, despite the effectiveness of the patented process in purifying ammonia, a number of practical difficulties related to the 100-150xc2x0 C. operating temperature of the getter alloy prevent significant industrial use of this method. First, St 707 decomposes ammonia at these elevated temperatures. Second, if the ammonia is contaminated with large concentrations of oxygen, then oxygen absorption by St 707, which is extremely rapid at the operating temperature of the getter alloy, can lead to an exothermic autocatalytic reaction that can explosively destroy the gas purification device. Finally, gas purification processes that operate at ambient temperature (i.e., at or about room temperature or 25xc2x0 C.) are preferred in the semiconductor industry because of cost considerations and engineering simplicity.
Therefore, a process that would remove oxygen from ammonia at about room temperature is highly desirable.
The present invention provides, in one aspect, a method that removes oxygen from oxygen contaminated ammonia to yield ammonia that is substantially oxygen free at low temperatures. Thus, the present process will be seen to provide a method for deoxygenating ammonia that avoids the above described problems with current methods that remove oxygen from ammonia Furthermore, the present invention offers significant cost and design advantages over the prior art.
In one embodiment, the present invention provides a method that removes oxygen from oxygen contaminated ammonia that comprises contacting the impure ammonia with a getter material that includes iron and manganese that sorbs oxygen to provide ammonia substantially free from oxygen at moderate temperatures. In one particular embodiment, the process of contacting the ammonia with the getter material takes place at about 25xc2x0 C. In another embodiment, the weight ratio between iron and manganese is about 7:1. In yet another embodiment, the getter material is deposited on an inert support of specific surface greater than about 100 m2/g.
In another embodiment, the present invention provides a method that removes oxygen from ammonia that comprises contacting the impure ammonia with a getter material that includes iron and manganese that sorbs oxygen and with a drying agent that absorbs water to yield ammonia that is substantially free from oxygen and water impurities. In one particular embodiment, the process of contacting the ammonia with the getter material takes place at about 25xc2x0 C. In another embodiment, the weight ratio between iron and manganese of the getter material is about 7:1. In yet another embodiment, the getter material is deposited on an inert support of specific surface greater than about 100 m2/g.
In still another embodiment, the present invention provides an apparatus consisting of a gas inlet, gas purification chamber and gas outlet where impure ammonia enters through the gas inlet, contacts a getter material which includes iron and manganese that sorbs oxygen in the gas purification chamber and exits through the gas outlet substantially free of oxygen. In one particular embodiment, the temperature of the gas purification chamber is maintained at about 25xc2x0 C. In another embodiment, the weight ratio between iron and manganese of the getter material is about 7:1. In yet another embodiment, the getter material is deposited on an inert support of specific surface greater than about 100 m2/g. In one embodiment, a drying agent is co-mingled with the getter material in the gas purification chamber to remove water from ammonia. In another embodiment, the drying material is physically separated from the getter material in the gas purification chamber. In still another embodiment, the drying agent and the getter material are present in separate, connected gas purification chambers.
In yet another embodiment, the present invention provides a method for producing a semiconductor device with high purity ammonia that comprises contacting oxygen contaminated ammonia with a getter material that includes iron and manganese that sorbs oxygen to provide ammonia substantially free from oxygen; introducing the purified ammonia into a semiconductor wafer processing chamber, and processing a semiconductor wafer in the processing chamber to yield a semiconductor device. In one particular embodiment, the process of contacting the ammonia with the getter material takes place at about 25xc2x0 C. In another embodiment, the weight ratio between iron and manganese of the getter material is about 7:1. In yet another embodiment, the getter material is deposited on an inert support of specific surface greater than about 100 m2/g. In still another embodiment, the ammonia is also contacted with a drying agent prior to entering the semiconductor processing chamber.
These and other aspects and advantages of the present invention will become more apparent when the Description below is read in conjunction with the accompanying Drawings.